This invention relates to optical lithography and, more particularly, to apparatus and methods for achieving short-wavelength optical lithography adapted for fabricating high-quality fine-line semiconductor devices.
As feature sizes on semiconductor integrated circuit devices approach one micrometer (micron) and below, extensive efforts are being directed at developing lithographic equipment capable of pattern delineation at these dimensions. Electron-beam, ion-beam and X-ray lithography have demonstrated patterning capabilities that extend into the submicron-line region. But workers in the art have recognized that there are obvious incentives for also trying to push the currently dominant technology (optical lithography) into this fine-line region. Such an effort, if successful, would have the potential of providing a basis for retrofitting or modifying expensive existing equipment to give it significantly better patterning capabilities.
It is known that the resolution limit (L.sub.min) for equal lines and spaces in an optical imaging system can be expressed as EQU L.sub.min =K.lambda./NA (1)
where K is a constant whose value is typically between 0.4 and 1.0 depending on processing and illumination conditions and resist characteristics, .lambda. is the wavelength of the exposing radiation and NA is the numerical aperture of the projection optics.
It is apparent from (1) that the minimum printable feature can be reduced by decreasing .lambda. or by increasing NA. But, since the depth of focus of the system varies inversely as (NA).sup.2, it is usually preferable in a practical high-resolution system to achieve the desired L.sub.min by reducing .lambda. rather than increasing NA.
Accordingly, wide interest exists in optical lithography at wavelengths below the conventional 4000 Angstrom unit (.ANG.) region. It was recognized that systems operating in this region (which includes the so-called deep-ultraviolet or deep-UV portion of the electromagnetic spectrum) would have the potential for patterning extremely fine lines (for example, 0.5-micron lines). Moreover, it was recognized that such a high-resolution system, if employed in an application where submicron lines are not needed in practice, would have the capability to be adjusted to exhibit an advantageously large depth-of-focus characteristic for patterning coarser lines. For these and other reasons, the development of short-wavelength (for example, deep-UV) lithography is viewed as an important step in the realization of high-quality fine-line integrated circuit devices.